Such control circuits are used in the most varied technological fields. They are utilized in particular in automotive technology. The removal of toxic substances in the exhaust gas of internal combustion engines, for example, is based on control circuits such as a controlled Lambda probe, in which the actuating variable, i.e., the pump current, represents the useful signal. The controlled variable, on the other hand, is the Nernst voltage, which is adjusted to a predefined value (450 mV). If this value of the Nernst voltage has been reached, the exhaust gas has a value of Lambda=1. The Lambda value is determined via the actuating variable, i.e., the pump current, in a manner that is known per se. Interference which influences the control may arise in such a control circuit, it specifically being the case that the controlled variable, that is to say, the Nernst voltage in the case of a Lambda probe, is adversely affected as well. For example, an interference quantity which is not related to the useful signal or the controlled variable, may be coupled in, e.g., via the connecting lead such as in a cable tree. Such a coupling can result from interference, which is produced by piezo injectors in Diesel engines, for instance, or it may be created in some other manner or possibly simply be due to the effects of EMC interference.
Such interference is characterized, for example, by a faster signal rise or drop time in comparison with the controlled variable, so that the interference is changing more rapidly per time unit than the controlled variable itself. In a control circuit such interference is not readily distinguishable from a control deviation. It may therefore cause a change in the actuating variable, and thus have a negative effect on the useful signal, that is to say, falsify the signal. Essentially three measures are known from the related art for removing such interference-causing influences. For one, the useful signal is filtered. Such filtering reduces the fault of the useful signal resulting from the coupled interference. A second measure is filtering of the controlled variable. This reduces the amplitude of the interference variable. Finally, extreme slowing of the control circuit is an option as well, e.g., by a lower P-component of the control characteristic. A disadvantage of these conventional methods is that damping of the interference is invariably accompanied by damping of the useful signal, or by a loss of dynamics of the useful signal.
Therefore, the present invention is based on the objective of further developing a method for suppressing interference in a controlled variable of the type described above, in such a way that interference is not only able to be identified, but the interference is suppressed and minimized in such a way that the effect on the useful signal is minimal and the dynamics of the interference-free useful signal are retained, if possible.